Can't help you there---bikini's (if I were to ever wear them again which probably won't happen)--I always had to buy larger b/c I wanted the ladies to stay well covered. So that would be my first guess.
As far as "athletic" swimsuits--I always have to buy smaller--b/c they are supposed to fit snug for the water-dyamics (excuse my ignorance--I do mean the water equivilant of aerodynamics).
I think it is par for the course--if you don't want to wear your suit dental floss style--you will inevitably be forced to purchase a larger size.
I buy a bigger swimsuit size because I don't want to tug. I'm a 10 right now in regular clothes (sometimes 8) and my last swimsuits were 12s. I bought a bikini too and am working on looking better in it! I bought the little skirt to go with it too which I will wear for now!